1. Field of the Invention
This invention relates to free radical solution polymerization, and, more particularly, to polymerization of vinylpyrrolidone (VP) to polyvinylpyrrolidone (PVP) polymer using a predetermined initiator system to control the molecular weight (MW), the molecular weight distribution (MWD), and the residual VP monomer level of the PVP product.
2. Description of the Prior Art
Free radical solution polymerization is used widely in industry to prepare homopolymers and copolymers. See, for example, U.S. Pat. Nos. 4,433,112; 4,554,311; 4,058,655; 4,053,696; 4,330,451; 3,862,915; and 4,886,861. In such process, the monomer, for example, vinylpyrrolidone, the solvent and the free radical initiator, are mixed and reacted at a selected temperature for a given period of time. Control of both the reaction temperature and viscosity of the reactants during the process has been considered essential in order to obtain a polymer of predetermined composition, MW, and viscosity, and having a narrow MWD.
It is also desired to carry out a free radical solution polymerization process so that the residual monomer content is quite low, preferably less than 0.1%. Polymers having a low residual monomer content can be made by adding the initiator in stages, or continuously, during polymerization, or by providing a booster amount of the initiator in the final stage of the process. However, these techniques require rather complicated manufacturing procedures. Furthermore, the viscosity of the reaction mixture increases as polymerization proceeds; accordingly, it was difficult to intimately mix such successive portions of the initiator to the viscous mass which resulted in a localized high concentration of free radicals and a considerable variation in the MW, MWD and viscosity of the final product.
For these reasons, a single initiator is preferred in free radical solution polymerizations. Unfortunately, the resultant initiator concentration after 99% of the monomer has been converted to polymer is too low to effectively reduce the residual monomer level from 1% to less than 0.1%, and in a timely manner.
Accordingly, it is an object of this invention to provide a process for free radical solution polymerization of monomers to provide polymers of predetermined MW, a narrow MWD, a high viscosity, and a low residual monomer content.
A particular object herein is to provide a process for free radical solution polymerization of vinylpyrrolidone to polyvinylpyrrolidone having a K-value of about 30-150, an MWD of 3-6, and a residual monomer content of less than 0.1%.
These and other objects and features of the invention will be made apparent from the following more particular description.